Others have previously recognized that film electrodes on zirconia exhaust gas sensor bodies should be porous and adherent. Some electrodes formed by sputtering are inherently porous as deposited. Extremely high surface areas are obtained if the film electrodes are sputtered by the process described and claimed in U.S. Ser. No. 089,264, entitled "Exhaust Electrode Process for Exhaust Gas Oxygen Sensor", filed Oct. 29, 1979, now U.S. Pat. No. 4,244,798, in the names of T. J. Gold, F. L. Kennard, III, P. C. Kikuchi and R. V. Wilhelm, Jr. Evaporated film electrodes are not necessarily porous as formed. However, they can be made porous by appropriate heat treatment, as for example as described in U.S. Pat. No. 3,978,006 Topp et al.
Pretreating the zirconia substrate to enhance porosity and adhesion of a film electrode is described in U.S. Pat. No. 4,135,040 Thornton. Texturizing the zirconia surface should enhance electrode adhesion. Such a technique is presumably primarily applicable to film electrodes that are not inherently high in porosity and/or surface area as normally deposited. However, it may enhance characteristics of film electrodes that are inherently high in porosity and/or surface area as deposited. I have found an etching process and an etchant for zirconia that provides a high degree of zirconia texturing from both a macro and a micro standpoint. It is particularly effective in improving performance characteristics of evaporated catalytic film electrodes.